Recently, a composition being curable through the irradiation of an active ray such as ultraviolet has been used in a broad range of fields such as coating materials, UV-curable and thermosetting coatings, molding materials, adhesive agent, ink, light-sensitive films, optical materials, stereo lithography materials, printing plate materials, dental materials, polymer battery materials and polymer materials. For example, as optical materials, the composition has been used for optical lenses, coating materials such as a film, clad materials for optical fibers, optical fibers and optical adhesives for optical lenses and the like.
The higher the requirements for high performance of the above-mentioned optical materials and electronic materials for use in each field, the higher the requirements for each performance of the curable composition in terms of reactive property, curing property, optical property of the cured product such as transmittance and refractive index, adhesiveness to a substrate, heat resistance, etc.
A curable composition comprising a thiol compound is known as one of these light-curable compositions.
Such curable compositions are one-component or two-component light-curable compositions, and cured within a period of a few seconds to a few minutes through radical polymerization of a compound containing a polyethylenically unsaturated double bond and a thiol compound by light irradiation. While these conventional polyene/polythiol-based light-curable compositions are excellent in workability and curability, they have low stability and become increasingly viscous and tend to cause gelation when they are kept in a liquid state before use. And the light-cured products produced by a composition kept in such a state have a problem of low heat resistance.
The invention of JP-A-2003-226718 (Patent Document 1) relates to a light-curable composition containing a specific polythiol, at least one polyene compound and a radical photopolymerization initiator. The curable composition is characterized in being capable of obtaining a cured product having high refraction index and high hardness by containing sulfur atoms in the composition.
The invention of JP-A-2003-277505 (Patent Document 2) relates to a light curable resin composition containing a compound having a bromine-substituted aromatic ring as well as polyene and polythiol, which enables to obtain high refraction index.
In JP-A-2003-226718 and JP-A-2003-277505 mentioned above, a curable composition which enables to obtain a cured product having high refraction index but has unresolved issues in terms of stability, reactive property, cure shrinkage factor and adhesiveness is obtained by containing sulfur atoms.
JP-A-2001-26608 (Patent Document 3) discloses a light-curable resin composition containing polyene, a photopolymerization initiator, and metal ions of 50 ppm or less. The composition is characterized in obtaining storage stability of the light-curable resin composition by reducing the amount of the metal ion.
JP-A-2004-149755 (Patent Document 4; WO 03/72614) discloses a photopolymerization initiator composition comprising a thiol compound containing a mercapto group having a specific substituent and a photopolymerization initiator. The invention is characterized in obtaining a photosensitive composition having high sensitivity and excellent storage stability.
However, while the invention of JP-A-2001-26608 and JP-A-2004-149755 enables to improve stability and refraction index of the composition, it still has unresolved issues in terms of adhesiveness, cure shrinkage factor, heat resistance and optical property which are required for the use as coating materials, adhesive agent and other electronics materials.
Additionally, JP-A-2004-264435 (Patent Document 5), JP-A-2005-195977 (Patent Document 6) and JP-A-2005-308813 (Patent Document 7) respectively discloses:                a photosensitive composition for color filters comprising a compound having an ethylenically unsaturated double bond, a photopolymerization initiator, a pigment and a specific thiol compound;        a curable resin composition for liquid crystal display elements comprising episulfide resin and a hardener; and        a curable resin composition for liquid crystal display elements comprising curable resin and a hardener.            [Patent Document 1] JP-A-2003-226718    [Patent Document 2] JP-A-2003-277505    [Patent Document 3] JP-A-2001-26608    [Patent Document 4] JP-A-2004-149755    [Patent Document 5] JP-A-2004-264435    [Patent Document 6] JP-A-2005-195977    [Patent Document 7] JP-A-2005-308813